Generally, Wireless Personal Area Network (hereinafter referred to as ‘WPAN’) technology is one of the core technologies for ubiquitous networks in which persons, computers and objects are connected to each other through a single structure. A representative example of the application of WPAN technology is a sensor network, which functions to combine an existing sensor network environment with an actual physical environment.
A sensor network occupies an important role in realizing the ubiquitous generation. In particular, when ubiquitous networking is implemented in the home, it will provide a large incentive to construct national-level infrastructure, and thus the construction of a ubiquitous networking environment in the home is very significant. Further, home networking technology, which is one of the ubiquitous networking technologies, has emerged as a core technology for overcoming the serious situation in which existing markets for electric home appliances are rather depressed, in the ubiquitous generation, and a sensor network is central to such home networking technology.
It is predicted that a sensor network, expected to be composed of several hundreds or thousands of small sensor modules, will be applied to various fields, such as remote monitoring in intelligent home networks, automatic manufacturing process control, the administration of warehouse and physical distribution, remote patient monitoring in ho spitals, and security systems for break-in detection.
Meanwhile, a WPAN can be used to implement such technology, and is characterized in that it has advantages, such as a transmission range of less than 10 m, low power consumption, and a size small enough to be mounted in a sensor or the like. Of such WPAN technologies, technology that is currently attracting attention includes ZigBee, which is low-speed and low-power WPAN technology. However, current ZigBee technology is limitedly applicable to networks and suffers from the instability of networks.
FIG. 1 is a diagram showing a transmission interval between beacons transmitted by a single node in ZigBee. A single node has a period during which data is transmitted or received after transmitting a beacon, and then has a sleep period in order to reduce power consumption. The intervals at this time are regularly designated and are equally applied to a next beacon transmission period, a next data transmission/reception period, and a next sleep period. That is, the beacon transmission period, the data transmission/reception period, and the sleep period are repeated at regular intervals.
During the sleep period, the transmission of data is possible, but the reception of data is impossible. The transmission of a beacon is required in order to connect a node below a reference node, so that the last node present in the configuration of a network does not require the transmission of a beacon after a certain period of time has elapsed.
FIG. 2 is a diagram showing the status of the transmission of beacons between two nodes occurring at the time of transmitting beacons between a first node and a second node. The first node transmits a beacon to the second node and transmits a time slot, ranging from the transmission of the beacon to the transmission of the next beacon, to the second node. The second node, having received the time slot, transmits its own beacon in the start section of the period, which is not used by the first node, in order to avoid collisions with the beacon transmitted by the first node.
Meanwhile, one of the main functions of a ZigBee network protocol is to assign logical addresses to construct a cluster tree. The cluster tree is formed in such a way that a ZigBee coordinator is set to the highest root and all nodes present in the ZigBee network are connected to the ZigBee coordinator in a lower tree structure. When the ZigBee network is formed and the tree is constructed, logical addresses that enable respective devices to be distinguished from each other and that allow data transmission routes or the like to be determined are assigned to respective nodes by their parent nodes.
Several terms are described below to describe conventional methods of assigning addresses. First, Cm is the maximum number of child nodes that a parent node can have. That is, this means that a maximum of Cm devices can be connected to a single device. However, the devices defined by Cm cannot be connected to lower devices any more. These devices are designated as ‘end devices.’
Lm is the maximum level number or the depth of a tree structure. FIG. 3 illustrates an example of Lm. That is, when Lm, is 2, the depth of devices to be connected may extend up to two hops, as shown in FIG. 3.
Rm is the number of lower devices connected to a reference node, similar to Cm. However, unlike Cm, devices defined by Rm have the function of connecting other devices below the lower devices. Such a function is called a coordinator function.
Since the above values Cm, Lm and Rm are determined, the infinite extension of sensor network devices can be limited. When a single service model is selected and a sensor network suitable for the service model is implemented, the number of devices suitable for the service model can be limited. That is, the values can be set as identical values for devices belonging to the same WPAN.
Meanwhile, a ZigBee coordinator determines the maximum number of child nodes “Cm” that each parent node can have, and the maximum level number or depth of a tree structure “Lm” so as to assign logical addresses when a ZigBee network is formed. Each parent node calculates a block size and an address block value (Cskip) on the basis of the values “Cm” and “Lm,” and is assigned a logical address block, which will be used by the child nodes thereof, by the ZigBee coordinator.
The address block value denotes the number of other lower devices that can be connected to a reference device in a number corresponding to the address block value. That is, when an address block value is 0, even a device functioning as a coordinator cannot assign an address value to devices below it any further, which means that no lower device can be connected to the device.
FIG. 4 is a diagram showing an equation for obtaining an address block value Cskip (d), which shows that, for a reference device having a depth of d, the number of lower devices that can be connected to the reference device and that include a lower device having a depth of d+1 can be detected. Further, there is no need to assign an address block to a device functioning as an end device because the end device does not perform the function of connecting a lower device thereto. Therefore, the end device is assigned only a single address to be used by the end device itself.
Through the equation shown in FIG. 5, an address value to be assigned to an end device that is connected in an n-th order can be obtained. Through this method, addresses can be assigned between WPAN devices formed based on a service model. In the equation of FIG. 5, An is the address of an end device that is connected in an n-th order among end devices, Aparent is the address value of the higher device of a reference parent device desired to be connected, and Cskip (d) is the address block value of the higher device of the reference device desired to be connected.
In order to show an example of an address block assignment method, the configuration of a WPAN is intended to be implemented by assigning addresses to a service model using values such as Cm=4, Rm=3, and Lm=3.
First, the maximum number of devices that can be connected to a reference device must be obtained using the reference device that was generated first. In the case of the reference device that was generated first, the depth thereof is d=0, so that an address block value, as shown in FIG. 6, is obtained using FIG. 4. Through this equation, address block values such as Cskip (1)=5, Cskip (2)=1, and Cskip (3)=0 are obtained. That is, this service model shows that a device having a depth of 1 can be connected to a maximum of five devices having a depth of 2 or more, a device having a depth of 2 can be connected to a single device having a depth of 3, and a device having a depth of 3 functions as an end device that cannot be connected to any more devices.
FIG. 7 is a diagram showing an example of the configuration of a network formed using the above-described embodiment.
In FIG. 7, since the first device has an address block value of Cskip (1)=5, the number of lower devices that are connected thereto, including the second device, cannot exceed 5.
The above-described address assignment method does not seem to have any special problem in the configuration of a network, but, in practice, there are many cases in which a great number of devices may exist in a small space in the case of a sensor network. Accordingly, although the total number of address blocks required for the configuration of the network of FIG. 7 is 17, a node having an address value of 52 exists, as shown in FIG. 7, and thus it can be seen that distribution efficiency is deteriorated.
Further, since address values used in ZigBee technology, which is widely utilized as sensor network technology for wireless personal communication, are 16 bits, addresses can be assigned to a maximum of 65,536 devices in a single WPAN area. However, the existing address assignment method is problematic in that, since the number of devices connected to a reference device is calculated in advance, and the reference device has a number of address blocks corresponding to the calculated number of devices, the reference device unnecessarily wastes its own address blocks if no device is connected to the reference device, even if the reference device has a great number of address blocks.
Further, another coordinator may not permit connection any further when all address blocks assigned thereto are exhausted. That is, balance in address distribution, which allows as many devices to attempt connection as possible in the WPAN area by assigning a large address block to a device to which a large number of devices is connected, is excluded.
Consequently, this interferes with the connection between various sensor devices in a ubiquitous sensor network, and the number of allowable devices is also limited when the above-described values Cm, Rm, and Lm are designated, thus resulting in a decrease in scalability and efficiency between different devices which do not belong to a designated service model. As a result, the existing method causes a problem of a change in the service model itself, which does not conform to ubiquitous networking, in which various types of service must coexist.